1. Field of the Invention
The present invention relates to low-viscosity reaction products of polyisocyanates which contain activated groups which react, with polymerization, with ethylenically unsaturated compounds on exposure to actinic radiation, to a process for preparing them and to their use in coating compositions.
2. Description of the Prior Art
The curing of coating systems which carry activated double bonds by actinic radiation, such as UV light, IR radiation or else electron beams, is known and is established in industry. It is one of the most rapid curing methods in coating technology.
Because of the environmental and economic requirements imposed on modern coating systems, that they should use as little organic solvents as possible, or none at all, for adjusting the viscosity, there is a desire to use coatings raw materials which are already of low viscosity. Known for this purpose for a long time have been polyisocyanates with an allophanate structure as are described, inter alia, in EP-A 0 682 012.
In industry these substances are prepared by reacting a monohydric or polyhydric alcohol with excess aliphatic and/or cycloaliphatic diisocyanate (cf. GB-A 994 890, EP-A 0 000 194 or EP-A 0 712 840). This is followed by removal of unreacted diisocyanate by means of distillation under reduced pressure. According to DE-A 198 60 041 this procedure can also be carried out with OH-functional compounds having activated double bonds, such as hydroxyalkyl acrylates, although difficulties occur in relation to the preparation of particularly low-monomer products. Since the distillation step has to take place at temperatures up to 135° C., in order to be able to lower the residue isocyanate content sufficiently (<0.5% by weight of residue monomer), it is possible for double bonds to react, with polymerization, under thermal initiation, even during the purification process, meaning that ideal products are no longer obtained.
The preparation of low-monomer-content, allophanate-containing, polyurethane-based, radiation-curing binders is described in EP-A 0 867 457 and U.S. Pat. No. 5,739,251. These binders, however, do not carry activated double bonds but instead carry unreactive allyl ether groups (structure R—O—CH2—CH═CH2). It is therefore necessary to add reactive diluents (low molecular weight esters of acrylic acid), which introduce the required UV reactivity.
EP-A 0 825 211 describes a process for synthesizing allophanate structures from oxadiazinetriones, although no radiation-curing derivatives having activated double bonds are known. All that is mentioned is the use of maleate- and/or fumarate-containing polyesters; the possibility of radiation curing is not described.
U.S. Pat. No. 5,777,024 describes the preparation of low-viscosity radiation-curing allophanates by reacting hydroxy-functional monomers which carry activated double bonds with isocyanate groups of allophanate-modified isocyanurate polyisocyanates. The allophanate-bound radicals are saturated as a result.
The formation of allophanate compounds by ring opening of uretdiones with alcohols is known in principle as a crosslinking mechanism in powder coating materials (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419, and also US-A 2003 0153 713). Nevertheless, the reaction temperatures required for this purpose are too high (≧120° C.) for a targeted preparation of radiation-curing monomers based on allophanate with activated double bonds.
Historically the direct reaction of uretdione rings with alcohols to allophanates was first investigated for solventborne, isocyanate-free, 2K [2-component] polyurethane coating materials. Without catalysis this reaction is of no technical importance, owing to the low reaction rate (F. Schmitt, Angew. Makromol. Chem. (1989), 171, pp. 21-38). With appropriate catalysts, however, the crosslinking reaction between HDI-based uretdione curatives and polyols is said to begin at 60-80° C. (K. B. Chandalia; R. A Englebach; S. L. Goldstein; R. W. Good; S. H. Harris; M. J. Morgan; P. J. Whitman; R. T. Wojcik, Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium, (2001), pp. 77-89). The structure of these catalysts has not been published to date. Commercial products prepared by utilizing this reaction are also undisclosed to date.
In summary it may be stated that the preparation of low-viscosity radiation-curing allophanates by ring-opening reaction of alcohols carrying activated double bonds with uretdiones at temperatures below 100° C. is not disclosed in detail in the prior art.